1. Field of the Invention
This invention relates to an electrochemical measuring cell for determining the oxygen content in gases, especially in exhaust gases of internal-combustion engines or combustion chambers, having a solid, oxygen ion-conducting electrolyte with a first layer serving as the measuring electrode, and a second layer used as the reference electrode.
2. Description of the Prior Art
Electrochemical measuring cells of this type deliver, if their electrodes are acted upon by gases with different partial oxygen pressure, an electric voltage which is proportional to the logarithm of the ratio of the partial oxygen pressures. As the reference electrode is usually exposed to air, the partial oxygen pressure of which is known, the electric voltage delivered by the measuring cell is a direct measure for the partial oxygen pressure of the measurement gas present at the measuring electrode. If the gas to be measured contains, in addition to the oxygen, oxidizable components such as carbon monoxide, hydrogen or hydrocarbons, then the thermodynamic gas equilibrium is adjusted by the catalytic action of the measuring electrode, i.e., the oxidizable or combustible gas components are oxidized, i.e., late or post-combustion, by the oxygen present in the gas to be measured. The oxygen content of the measurement gas pretreated in this manner is determined by the measuring cell. The electric voltage delivered by the measuring cell changes in step-fashion if the gas to be measured changes from one composition which contains oxidizable components in excess, to a composition without such excess of oxidizable components or with excess oxygen; a change in the opposite direction causes the same voltage step.
This step-like change of the electric voltage occurs in the measurement of exhaust gases of internal-combustion engines or combustion chambers when the ratio of the combustion air to the fuel reaches the stoichiometric value. This ratio is characterized, as is well known, by an air number .lambda.=1. If the air number is .lambda.&lt;1, this means that the combustion takes place with a deficiency of air and no excess air or oxygen is present. With an air number .lambda.&gt;1, excess air or oxygen is present in the fuel/air mixture.
In one known electrochemical measuring cell, German Published Non-Prosecuted Application No. 2 502 409, from which the present invention starts, there is provided on the catalytically effective layer, which serves as the measuring electrode, a further catalytically active layer which serves as a coating. This design causes a change, which is as step-like and sudden or abrupt as possible, of the electric voltage delivered by the measuring cell in the vicinity of an air number of approximately .lambda.=1. While such a measuring cell is satisfactory for regulating or measuring a fuel/air mixture with a value of about .lambda.=1, it is not suitable for measurement or control outside this range, as there, the electric voltage delivered by the measuring cell depends only to a very slight degree on the air number.